package usaco;
/*
ID: toregg1
LANG: JAVA
TASK: barn1
 */

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Arrays;

public class barn1 {
	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new FileReader("usaco/barn1.in"));
		PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("barn1.out")));
		int rst = 0;
		
		String line = in.readLine();
		int maxBoards = Integer.parseInt(line.split(" ")[0]);
		int totalStalls = Integer.parseInt(line.split(" ")[1]);
		int cowsNum = Integer.parseInt(line.split(" ")[2]);
		
		int[] cows = new int[cowsNum];
		for (int i = 0; i < cowsNum; i++) {
			line = in.readLine();
			cows[i] = Integer.parseInt(line);
		}
		
		if(maxBoards > cowsNum){
			maxBoards = cowsNum;
		}
		
		Arrays.sort(cows);
		
		int[] largerPre = new int[cowsNum];
		if (cowsNum != 0) {
			largerPre[0] = 0;
			for (int i = 1; i < largerPre.length; i++) {
				largerPre[i] = cows[i] - cows[i - 1];
			}
			
			int len = maxBoards - 1;
			int[] insertPosition = new int[len + 1];
			int max = 0;
			for (int i = 0; i < len; i++) {
				for (int j = 0; j < largerPre.length; j++) {
					if(largerPre[j] > max && !arrHas(insertPosition, j)){
						max = largerPre[j];
						insertPosition[i] = j;
					}
				}
				max = 0;
			}
			
			insertPosition[len] = cowsNum;
			Arrays.sort(insertPosition);
			
			for (int i = 0; i < insertPosition.length; i++) {
				if(i == 0){
					rst = cows[insertPosition[i] - 1] - cows[0] + 1;
				}else{
					rst += cows[insertPosition[i] - 1] - cows[insertPosition[i - 1]] + 1;
				}
			}
		}
		
		
		
		System.out.println("rst: " + rst);
		out.close();
		System.exit(0);
	}
	
	private static boolean arrHas(int[] a, int i){
		for (int j = 0; j < a.length; j++) {
			if(a[j] == i)
				return true;
		}
		return false;
	}
}
